Current state of the art insulation compounds for medium and high voltage cables are based upon low density polyethylene (LDPE) containing about 2 weight percent (wt %) of peroxide. This basic formulation is generally enhanced for commercial use by the addition of a range of additives which includes the following: anti-oxidants, heat stabilizers, scorch retarders, cure boosters, voltage stabilizers and in the case of medium voltage cables, an additive to inhibit water treeing. The final commercial formulation is often a compromise between scorch, cure, heat ageing performance and electrical behavior.
The majority of crosslinkable polyethylene (XLPE) insulation compounds use high pressure/low density polyethylene (HPLDPE) and dicumyl peroxide (DCP) as a basis for the formulation. LDPE has good melt strength and shear thinning behavior which is required for cable processing, and it does not contain any metallic catalyst residues which can impact electrical performance of the cable. In general a tubular LDPE of approximately 2 melt index is the standard resin of choice for medium voltage power cable insulation.
The peroxide of choice is typically DCP which is a relatively low cost, low activation energy peroxide which can be effectively soaked or compounded into polyethylene to yield a fully curable system. DCP-based systems enable melt extrusion of the LDPE compound without extensive premature peroxide decomposition. In general peroxide levels of about 2 wt % are employed but the exact level depends upon the actual structure of the LDPE, particularly the level of vinyl unsaturation, and the presence of other additives such as stabilizers in the blend.
XLPE compounds also contain antioxidants, the most common of which are the thiophenolic stabilizers. These stabilizers give processing stability, long term ageing protection of the cable, and minimal interference with the peroxide crosslinking reaction.
To improve the scorch-cure balance of the XLPE composition, additives such as α-methyl styrene dimer can be added. This additive improves both the scorch performance of the XLPE composition and acts as a cure booster or co-agent to improve the final cure state of the crosslinked insulation.
In the case of medium voltage insulation formulations, often a water tree retardant additive is required, e.g., low levels (less than 1 wt %) polyethylene glycol to ensure water tree retardancy of the crosslinked ethylene-based polymer. In the case of high voltage compositions, voltage stabilisers such as aromatic amines can be added which impact the initiation and growth of electrical trees. Such defects are the cause of failure or breakdown of crosslinked ethylene-based polymer insulation in an actual service environment.
However in spite of the continued improvement of XLPE formulations for cable insulation, current compounds suffer from a range of limitations due to additive solubility limitations and the antagonistic interaction of the additives themselves.
The ethylene-based polymer, e.g., LDPE, used to make the cable insulation is typically made, stored and transported to the site at which it is converted into cable insulation in the form of pellets. These pellets often comprise one or more additives which are either mixed with the ethylene-based polymer before it is pelletized or subsequently added to the pellet, e.g., coated onto or imbibed into the pellet.
The biggest issue with XLPE compositions is the migration of the peroxide to the outside or surface of the polymer pellets during compound storage and/or transport. For example, the maximum solubility of DCP in LDPE is estimated at around 1 wt % at room temperature, well below the actual levels used commercially (about 2 wt %). Therefore commercial XLPE compositions suffer from significant peroxide migration issues, an effect which increases with time. Temperature also has a major impact, and the migration of DCP is thought to have a maximum at around 5° C. Temperature cycling such as that found during day/night cycles is also thought to increase the tendency of peroxide migration.
XLPE pellets which have migrated peroxide on the surface lead to a number of problems during cable production. This is due to the lower melting peroxide coating which can impact the pellet feeding process. The slippage and irregular feeding of the XLPE composition with migrated peroxide can lead to variation in cable core diameter and increased scrap cable generation. This is a significant problem for XLPE composition producers and cable manufacturers.
The migration of peroxide can be greatly reduced in the case of medium voltage tree retardant compositions which comprise significant levels of acrylate copolymer blended into the LDPE (WO 85/05216, U.S. Pat. No. 5,539,075 and US 2009/0029166). However the use of acrylate copolymers increases significantly the dissipation factor (tan delta) of the polymer composition rendering the copolymer approach ineffective for higher voltage insulations where dissipation factor should be kept as low as possible.
Therefore a need exists for an XLPE composition which is able to maintain scorch-cure performance, heat ageing performance and electrical performance while also reducing significantly the tendency for peroxide migration.